1. Field of the Invention
This invention relates to liquid crystal displays (LCD's) employing means for measuring touch position, and, more particularly to LCDs wherein such touch measuring means is integrated into a substrate of the LCD.
2. Related Art
Touch input systems which determine the location of an object or person touching a surface are utilized in a wide variety of application and require that the location of the touch be determined with a high degree of accuracy. Typically, these devices are transparent and are fitted directly over a computer display. Examples of such add-on touch screens which can be fitted over a CRT or flat panel display may be found in "You can touch this! Touch screens deliver multimedia to the masses" by C. Skipton, New Media, Feb. 10, 1997, p. 39-42. Add-on touch screens have used a variety of methods for determining the touch location. Typically, the surface of such a system includes a layer of substantially uniform resistivity and electrodes are connected to the edges of the surface. The electrodes are usually made of a material which is more conductive than the surface and are often silk-screened onto the surface in a specific pattern. A frequently used method of determining the location is to apply an electric potential in a first direction and then apply an electric potential across the surface in a second direction orthogonal to the first direction. As described in U.S. Pat. No. 3,591,718 to Shintaro et al. and U.S. Pat. No. 4,649,232 to Nakamura et al., the x and y location can be determined using a capacitively coupled probe (through an insulating layer) to measure the voltages. The location of a finger touch can be determined by monitoring the current flow through the electrodes since the body shorts the applied AC signal to ground but this requires that the stray currents in the system are very small due to the small capacitance to a finger. U.S. Pat. No. 4,293,734 to Pepper, Jr describes an alternate approach by i) applying power to all four sides with an AC signal and using the ratio of electrode currents to determine the finger touch location, or ii) grounding the corners and measuring and currents induced by a powered stylus at the corners. In U.S. Pat. No. 4,686,332 to Grenias et. al., an add-on touch screen is described that includes two spaced conductor planes. Finger touch position is determined by detecting the change in the capacitance of conductor planes due to the fingertouch. Finally, in U.S. Pat. No. 4,371,746, an add-on touch panel is described that includes a guard layer of conductive material beneath (or partially surrounding) a resistive touch surface. The guard layer of the add-on panel is energized with a signal that is the same amplitude and phase as the signal applied to the resistive touch surface in order to substantially reduce the effective capacitance to ground of the resistive touch surface.
A disadvantage of such add-on touch screens is that they increase the weight and size of the display unit, which is a strong demerit for use in portable applications such as notebook computers. In addition, communication of the device to the computer requires an available card slot or serial or parallel port adapter. These disadvantages can be greatly reduced by integration of the touch sensor into the LC display. For application to liquid crystal displays, especially for portable applications, a capacitive touch technique is most suitable because of the compact size and high transmissivity (85-90%). Resistive touch technologies require two layers of Indium Tin Oxide (ITO), rather than one, and have a transmissivity of only 55 to 75%.
Typically, for a color LC display, the glass substrate orientated towards the viewer has the color filter built onto the inner surface which contacts the liquid crystal material and the color filter substrate (CF) is bonded along the edges to a second glass substrate which contains a active (or passive) matrix for addressing the LC display. The color filter is typically composed of a black matrix material (such as layers of Cr and Cr oxide), polymer layers containing pigments or dyes, and an ITO layer which is used as a common electrode for the display (in an active matrix display, or for a passive matrix display, it is patterned into lines and used for addressing the display), see "Color filter for 10.4 in. diagonal 4096-color thin-film-transistor liquid crystal displays", T. Koseki et al. IBM J. Res. Develop. Vol. 36 No. 1 January 1992. Additionally, a photosensitive transparent overcoat layer is frequently used to planarize the pigmented regions prior to the ITO common electrode deposition as described in U.S. Pat. No. 5,278,009 to Tsutsumi et al.
There have been two reports on modification of the upper glass substrate (e.g., the substrate that includes the color filter) of an LCD to permit stylus input using capacitive sensing. In the publication by J. H. Kim et al. "A design of the position-sensitive TFT-LCD for pen applications", SID '97, p. 87-90, the conductive black matrix (BM) layer in the color filter was isolated from the ITO common electrode by an overcoat layer and an AC signal was applied to the BM. Compensation resistors were formed outside the array to linearize the electric field and the direction (X & Y) of the applied field was alternated and the stylus position determined by measuring the voltage capacitively coupled into the tethered stylus. It was noted that a large signal was induced on ITO common electrode layer, with an amplitude of almost half of the input signal to the BM.
In the publication by H. Ikeda et al., "A New TFT-LCD with Build-in Digitizing Function", ISW '97, p. 199-202, a 6.6 inch VGA reflective Guest-Host AMLCD is described which uses the ITO common electrode as a resistive layer for stylus input. An AC signal voltage gradient is applied alternately in the X or Y direction on top of the DC bias on the ITO common electrode. Some linearization of the field was performed by using separate Al strip electrodes along each edge which were connected to the active region by ITO resistors 0.25 mm wide.times.1 mm long on 2.5 mm centers. The A1 resistivity was adjusted for the best linearization without excessive power consumption. The field was measured capacitively from a tethered stylus and the location determined using data pre-measured for typical positions on the display and stored in the computer.
Neither of these integrated structures is appropriate for measuring the contact position of a portion of the human body (such as a finger or a toe) with a substrate of the LCD due to the fact that the capacitive coupling (i.e., the effective capacitance) between the signal layer and the ITO common electrode (Kim et al.) or the wiring in the TFT array (Ikeda et al.) is much stronger than the capacitive coupling (effective capacitance) between the substrate and the human body.
Thus, there is a need in the art to develop a structure integrated into a substrate of an LCD that is suitable for measuring the contact position of a portion of the human body (such as a finger or a toe) with the substrate.